This section provides background information related to the present disclosure which is not necessarily prior art.
A model of a typical ultrasonic plastic welder 100 is shown in FIG. 1. Typical components of ultrasonic plastic welder 100 include an ultrasonic stack 101 having an ultrasonic transducer 102, a booster 104, and an ultrasonic horn 106. As is commonly understood by those of skill in the art of ultrasonic welding, an ultrasonic horn is a metal bar that transfers mechanical energy from the ultrasonic transducer to the work piece. The ultrasonic horn is typically one-half wavelength long at the resonant frequency that is produced by the ultrasonic transducer. Electrical energy from a power supply 122 at a frequency of 20-60 kHz is converted to mechanical energy by the ultrasonic transducer 102. The mechanical energy converted in the ultrasonic transducer 102 is transmitted to an application 108, such as two parts 112, 114 that are to be welded together, through the booster 104 and the ultrasonic horn 106. The booster 104 and the ultrasonic horn 106 perform the functions of transmitting the mechanical energy as well as transforming mechanical vibrations from the ultrasonic transducer 102 by a gain factor. The ultrasonic welder 100 is controlled by a controller 124 that has inputs and outputs coupled to the applicable components of ultrasonic welder 100. It should be understood that power supply 122 can include controller 124 (as shown in FIG. 1) or controller 124 can be a separate device from power supply 122.
The mechanical vibration that results on a horn tip 110 is the motion that performs the task of welding the parts 112, 114 together, which in the following example are plastic parts. Horn tip 110 may be made of titanium or other high strength, hard material. The parts 112, 114 to be welded together are placed adjacent to the horn tip 110, such as being placed together on an anvil 120. The horn tip 110 is brought into contact with top part 112 to be welded. The ultrasonic horn 106 oscillates in a manner to impart vertical oscillations in the plastic pieces. That is, the ultrasonic horn causes oscillatory compression/decompression of the plastic parts with respect to each other causing surfaces of the plastic parts abutting each other at a weld interface to be heated, eventually melting together.
Most ultrasonic weld processes are processes controlled by a weld parameter. The weld parameter is a parameter that is used to determine when to stop the application of weld energy—the mechanical vibrations—to the parts being welded. In one example, the weld parameter is time in which case the weld process is a time controlled process where the mechanical vibrations are applied to the parts being welded for a predetermined period of time to which the weld parameter was set. In another example, the weld parameter is energy in which case the weld process is an energy controlled process where the mechanical vibrations are applied to the parts being welded until a predetermined amount of energy to which the weld parameter was set has been utilized. In another example, the weld parameter is a collapse distance in which case the weld process is a weld by collapse distance process where the mechanical vibrations are applied to the parts being welded until they have collapsed together by a predetermined distance to which the weld parameter was set. In this regard, collapse distance is often determined by the distance that the ultrasonic horn tip moves toward the anvil once the two parts being welded begin to melt together. It should be understood that other weld parameters can be utilized, such absolute distance, comparative distance, frequency, amplitude, amplitude profile, force, force profile, power output, temperature, and approach speed.
Ultrasonic welders used in automated installation are typically employed under stable operating conditions for best operation. Stable operating conditions are achieved when the stack has reached a stable temperature after a ramp-up phase. The ramp-up phase is the time after being idle or shutdown and the weld installation welds the first parts in series production of the production cycle until the time at which the stack has carried out a enough welds to reach a stable temperature. At the beginning of the ramp-up phase the stack has a lower temperature, for example the temperature of the environment where the welder is installed. The stack warms up as the ramp-up phase continues until it reaches a stable temperature. The ramp-up phase ends when the stack reaches a stable temperature.
A steady state condition is when the ultrasonic stack has reached a nominal stable temperature which is typically considered as occurring once ultrasonic welder has performed a sufficient number of weld operations for the ultrasonic stack to have reached the nominal stable temperature. However, the ultrasonic welder typically is never at a true steady state condition as the temperature of its ultrasonic stack can still vary even after reaching the nominal stable temperature such as due to changes in the ambient temperature of the area in which it is located. For example, the ambient temperature of an area of a factory in which the ultrasonic welder is located may be several degrees lower during the early morning than during the late afternoon. This difference in ambient temperature will affect the temperature of the ultrasonic stack even though the ultrasonic welder has been running continuously throughout the day.
As the temperature of the ultrasonic stack varies, so too does the heat energy in the ultrasonic stack. An increase in temperature of the ultrasonic stack results in increased heat energy in the ultrasonic stack and conversely, a decrease in temperature of the ultrasonic stack results in decreased heat energy in the ultrasonic stack. Variations in heat energy in the ultrasonic stack can cause variations in weld results of the ultrasonic welder. In this regard, the weld parameter used for the weld process is typically set for when the ultrasonic stack is at the steady state condition. Thus, as the heat energy in the ultrasonic stack varies due to variations in temperature of the ultrasonic stack, there can be resulting variations in weld results.